Battery system

ABSTRACT

Battery powered equipment is provide with a controller which monitors the voltage supplied by the battery. Should the battery voltage drop below a preset level when the equipment is inactive, the controller disconnects non-essential loads of the equipment from the battery to conserve what charge remains in the battery. When used with a motor vehicle the controller may be tied into the security system and disable the ignition during inactive periods unless the proper reactivation signal is received.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/074,629 filed Feb. 13, 1998.

FIELD OF THE INVENTION

[0002] The present invention relates generally to battery poweredelectrical systems, such as in motor vehicles; and more particularly toa control system for monitoring and maintaining the charge of thebattery while the electrical system is in an inactive state.

BACKGROUND OF THE INVENTION

[0003] Automobiles and other combustion engine powered vehiclestypically employ an electric motor to start the combustion engine. Forthat purpose, the electric motor is coupled to a starting circuit whichgenerally receives electrical power from an on-board storage battery.The starting circuit selectively couples electrical energy from thebattery to the starting motor that operates to cycle the engine toinitiate sustained operation. In common vehicle applications, thebattery also provides electrical energy to a variety of electric powerconsuming devices, such as engine control electronics, lights, andvehicle accessories.

[0004] Traditional batteries for these applications, often referred toas starting, lighting and ignition (SLI) batteries, are multi-cell,lead-acid batteries. That is, the batteries are constructed from leadplates pasted with active material and arranged into stacks. Thosestacks are inserted into partitioned cell compartments of a batterycontainer, electrically interconnected, and flooded with clilute acidelectrolyte. SLI batteries of this construction are more than adequatefor providing the relatively high power demand required of enginestarting, as well as the relatively low power demand to maintainelectrical accessories during both vehicle operation and periods ofnon-operation. However, because of the seemingly disparate functions theSLI battery is required to perform, short duration high-power output andlong duration low-power output, the battery design can not be optimizedfor performing either of these tasks. An additional drawback of thesebatteries is relatively low specific energy (kilowatt hour/gram, kWh/g)as compared to other battery constructions owing to the weight of thelead plates and the liquid electrolyte.

[0005] There has been suggested a battery system for vehicle use whichincludes two batteries. A first battery in the system, a startingbattery, is optimized to start the engine by being specifically designedfor short duration, high-power output. A second battery in the system, areserve battery, is optimized to operate and maintain non-startingelectrical loads, such as for vehicle accessories. An advantage of sucha system is that the starting battery may be made smaller and lighteryet capable of providing a high power output for a short period of time.In addition, the reserve battery may be made smaller and lighter yetcapable of satisfying the relatively low power requirements of vehicleaccessories. In combination, the two batteries may require less spaceand weigh less than a single traditional SLI battery.

[0006] A limitation of a two battery system lies with maintaining thecharge of both batteries. Typically, the vehicle includes avoltage/current regulation device which regulates the output of thealternator in response to the charging needs of the SLI battery and thevehicle electrical loads. In the dual battery system, each battery typedelivers power and accepts charge at a different rate. For example, thestarting battery delivers power at a very high rate and likewise acceptscharge at a high rate. In contrast, the reserve battery delivers powerat a lower rate and accepts charge at a lower rate. Moreover, it willtypically be the case that each battery will be at a differentstate-of-charge, hence requiring different charge maintenance.Additional advantages may also be attained by selectively coupling ordecoupling the batteries during inactive, starting and operationalperiods of the vehicle. However, careful management is required so asnot to damage either the vehicle electrical system or the dualbatteries.

[0007] Another problem encountered with battery powered equipment isbattery drain during periods of inactivity. For example, a motor vehiclemay sit parked for several weeks or months. In that situation a leakagecurrent or current drawn by accessories left turned-on can drain thebattery to a point where the remaining charge is insufficient to startthe engine. Thus it is desirable to provide a control mechanism thatresponds to a period of inactivity by disconnecting nonessential loadsfrom the battery.

SUMMARY OF THE INVENTION

[0008] The present battery system is particularly adapted for use in avehicle which has an electric motor for starting an engine, analternator driven by an engine to generate electricity, and accessoryelectrical loads. The battery system has a first battery for selectivelypowering the electric motor to start the engine and a second battery tooperate and maintain accessory electrical loads. A charge maintenancedevice connects the first battery to the second battery for the purposeof maintaining the charge of the first battery at a predefined level. Acontroller monitors the voltage level of the first battery to sense whenthe battery charge level has decreased to a level at which recharging isneeded. At that time the controller operates the charge maintenancedevice to recharge the first battery from the second battery.

[0009] In the preferred embodiment of the battery system a chargingswitch is provided which selectively connects the first battery to thealternator. The controller activates the charging switch in response tovoltage across the second battery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a block diagram showing incorporation of the presentinvention into a dual battery electrical system of a motor vehicle; and

[0011]FIG. 2 is a block schematic diagram of the circuitry for thecharge maintenance device shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention is described in terms of a preferredembodiment adapted for use in a dual-battery based vehicle electricalsystem. The batteries in the system provide electrical energy forvarious vehicle operation functions and receive charging from thevehicle electrical system. It will be appreciated that the scope of theinvention is not limited to vehicle applications or dual batterysystems. For example, the invention may find application in a singlebattery system.

[0013] In various preferred embodiments of the present invention,battery control electronics, vehicle control electronics andcombinations of the these electronic control devices are utilized forbattery charge management and enhanced system performance. For example,the system is adaptable to automatically determine charge status of thebatteries in the system and to couple, as appropriate, the battery orbatteries with sufficient charge to operate essential vehicle electricalloads and to provide energy for starting. In addition, a preferredcharge management strategy reduces the potential for over-charging oneor more of the system batteries and yet maintains each of the batteriesat a ready state-of-charge. The control system also disconnectsnon-essential loads from the batteries when the battery voltage dropsbelow a defined level during periods of vehicle inactivity. These andother advantages and features of the present invention will beappreciated from the description of the preferred embodiment whichfollows.

[0014] Referring to FIG. 1, a vehicle electrical system 10 includes abattery subsystem which has a starting battery 14 coupled for providingelectrical energy to engine starting motor 22 through starter relaycontacts 24. Starting motor 22 is mechanically coupled to the engine ofthe vehicle (not shown) for starting the engine as is well known in theart. Starting battery 14 is preferably a high-rate battery, such as theone shown and described in commonly assigned U.S. patent applicationSer. No. 08/870,803 entitled: “Modular Electric Storage Battery” filedJun. 6, 1997, the disclosure of which is hereby expressly incorporatedherein by reference.

[0015] A reserve battery 20, which is preferably an absorptive glass mat(AGM) type construction with a high reserve capacity, is adapted toprovide a relative low-rate discharge for an extended period of time.The reserve battery 20 furnishes power to essential vehicle electricalloads 15.

[0016] The electrical system 10 also includes system controller 18coupled to both starting battery 14 and the reserve battery 20. Thecontroller 18 is a microcomputer with internal memory and input/outputports and executes a control program to perform the functions beingdescribed herein. Controller 18 governs the connection of the startingbattery 14 and the reserve battery 20 to electrical system 10, andparticularly to the essential vehicle loads 15 and other vehicle loads30, for selectively providing electrical energy during normal vehicleoperation and during inactive periods. The essential vehicle loads 15may comprise such devices as the vehicle engine/power train controller,safety system controller and the like which require power even duringperiods when the vehicle is not operating. Non-essential vehicle loads30 may include accessories such as interior lights, entertainmentsystems, convenience features and the like, which are not required to bepowered during inactive periods.

[0017] An alternator 21 also is connected to electrical system 10. Thealternator is mechanically driven by the engine in a manner that is wellknow in the art and during periods of vehicle operation generateselectrical energy for charging starting battery 14 and reserve battery20 under the supervision of controller 18. The alternator 21, pursuantto operation of controller 18, also provides electrical energy tovehicle loads 15 and 30, as well as ignition system 32 during normaloperation. The output of alternator 21 is controlled through fieldvoltage regulation or other suitable means responsive to the controller18 or the engine/power train controller (not shown) as is known in theart.

[0018] A charging switch, formed by contacts of relay 16, directlycouple the starting battery 14 and reserve battery 20. A chargemaintenance device 12, also referred to as a “charge pump”, is connectedin parallel with the relay contacts. The charge maintenance device 12under control of controller 18 couples energy from the reserve battery20 to the starting battery 14 to maintain the charge status of startingbattery. For example, energy may be channeled to the starting battery 14during periods when the vehicle is not being used or during periods ofoperation where the starting battery requires additional charge. Since arelatively small power draw from reserve battery 20 may be used tomaintain starting battery 14 at a substantially full state-of-chargewithout adversely effecting the charge status of reserve battery 20, theself-discharge characteristic of starting battery 14 may be overcome.

[0019]FIG. 2 illustrates a preferred embodiment of charge maintenancedevice 12 having a circuit 200 which provides milliampere current pulsesfrom reserve battery 20 to starting battery 14. The circuit 200 includesNAND gates 202, 212 and 214 which are operatively coupled to form apulse generator, Specifically the reserve battery 20 is coupled a firstinput of NAND gate 202 through transistor switch 238 which is operatedby the enable signal (EN) from the controller 18. A second input iscoupled to output of NAND gate 202 by resistor 204. A series combinationof resistor 208 and diode 206 is coupled in parallel with resistor 204and capacitor 210 couples the second input to circuit ground. Theconnection of components forms an square wave oscillator. That is, whenswitch 236 is closed, NAND gate 202 produces a periodic pulse train. Theprecise frequency of the pulse train is not critical to operation ofcircuit 200, but is preferably set at about 5-30 kilohertz (kHz).

[0020] The pulse train is buffered and amplified through NAND gates 212and 214 and coupled via a resistor network, including resistors 216 and218, to the gate of transistor 220. In the preferred embodiment,transistor 220 is a field effect transistor (FET), but it should beunderstood that any suitable switching device may be used withoutdeparting from the fair scope of the invention. The application of thepulse train alternately turns on and off transistor 220.

[0021] When transistor 220 is conductive, current flows from thepositive terminal 28 of reserve battery 20 through inductor 226,transistor 220 and resistor 224. This causes voltage to build up acrossthe inductor 226. In the non-conductive state of transistor 220, thevoltage built up across inductor 226 is discharged through a currentlimiting resistor 234 into the starting battery 14, thereby providing acharge maintenance current. Diode 228 prevents reverse current flow, andresistor 230 and Zener diode 236 provide a voltage dumping path whichprotects transistor 220 from excessive voltage. Zener diode 236preferably has a 15-16 volt reverse breakdown level thereby clamping thevoltage across inductor 226 at that level. Construction and operation ofthe charge maintenance device 12 is described in greater detail incommonly assigned U.S. patent application Ser. No. 08/932,950 entitled“Battery Charge Maintenance System and Method” filed Sep. 17, 1997 by aco-inventor of the present invention and the disclosure of which ishereby expressly incorporated herein by reference.

[0022] When the alternator is not producing electricity, the controller18 acts to open and close switch 238 for activating and deactivating thecharge maintenance circuit 200 to maintain the starting battery at agiven charge level. However, it is possible to allow circuit 200 tooperate continuously without adverse affect to either starting battery14 or reserve battery 20. Nevertheless, to maximize the standbycapability of the system the preferred embodiment of circuit 200 isactivated when starting battery 14 voltage falls below a predefinedthreshold, as will be described subsequently. For example, thecontroller 18 senses starting battery 14 voltage and when it falls belowapproximately 12.75 volts to close switch 238 activate the chargemaintenance device 12.

[0023] Once activated, controller 18 initiates a timer, and the chargemaintenance device 12 is allowed to operate for 6 to 24 hours dependingcapacity of the starting battery 14 and the ability of circuit 200 toprovide charge current to starting battery 14. At the conclusion of thetime period, switch 238 is opened deactivating charge maintenance device12. Controller 18 also can be adapted to sense when starting batteryvoltage exceeds a threshold value for deactivating the chargemaintenance device 12, or the controller may continuously activatedevice 12 in response to various operating conditions, for example,environmental conditions such as extreme ambient cold.

[0024] Referring again to FIG. 1, during normal starting of the motorvehicle engine when the batteries 14 and 20 are properly charged,charging relay 16 is de-energized so that the starter motor 22 ispowered only by the starting battery 14 when the starter relay contacts24 close. At this time, the controller 18 monitors the voltage acrosseach battery 14 and 20 via connections provided by conductors 23 and 25,respectively, to the positive terminals of the batteries. If thecontroller 18 senses that the voltage from the reserve battery 20 isbelow a given level during starting, the controller energizes chargingrelay 16 so that the starting battery 14 will be connected to supplypower to those other car loads 15. In this normal condition, other carloads 15 are powered by the reserve battery 20.

[0025] Once the engine starts, if the voltage provided to the car loads15 (i.e. the voltage at terminal 28) is 13.6 volts or more, thecontroller 18 energizes charging relay 16 so that the starting battery14 is charged by voltage from alternator 21. However, when the voltageprovided to the car loads 15 drops to 13.1 volts or less, the chargingrelay 16 is de-energized so that its contacts open terminating chargingof the starting battery 14.

[0026] The controller 18 also provides protection against the batteriesbecoming excessively drained during periods when the motor vehicle isinactive. To this end, the non-essential accessory vehicle loads 30 areconnected to the positive terminal 28 of the reserve battery 20 througha first MOSFET transistor 34, and the ignition circuit 32 is coupled tothat positive terminal 28 through a second MOSFET transistor 36. Thegate electrodes of first and second MOSFET transistors 34 and 36 areconnected to and operated by separate outputs of controller 18, therebyacting as power switches which govern application of electricity to theaccessory vehicle loads 30 and the ignition circuit 32.

[0027] When the driver parks the motor vehicle, the controller 18detects that the ignition switch 40 has been turned off and responds byactivating an internal timer. After a predefined period of time (e.g.two minutes) elapses, the controller 18 begins periodically measuringthe voltage provided by the reserve battery 20. Should that voltage dropbelow 12.2 volts the controller 18 turns off the first MOSFET transistor34 thereby disconnecting power from being applied to non-essentialaccessory loads 30. This stops further power consumption by such loads,as a dashboard clock, which otherwise would drain the reserve batteryfurther. This disconnection conserves the remaining battery charge.

[0028] Upon exiting the vehicle, the driver may press a button of a keyfob 42 of a type used in keyless entry systems. That action causes thefob 42 to transmit a radio frequency (RF) signal 44 to a receiver 26 inthe vehicle to indicate that the security system for the vehicle shouldbe armed. In response, the RF receiver 26 sends a security system armedsignal to the controller 18, which responds by turning off the secondMOSFET transistor 36 disconnecting application of electrical power tothe ignition circuit 32. This action prevents a car thief from beingable to start the car, even if the thief is able to operate the ignitionswitch 40.

[0029] Upon returning to the vehicle, the driver presses another buttonof the key fob 42 which transmits a radio frequency (RF) signalindicating that the security system should be disarmed. The receipt ofthis second RF signal is communicated by the receiver 26 to thecontroller 18 which responds by turning on both first and second MOSFETtransistors 34 and 36, thereby powering non-essential accessory loads 30and the ignition circuit 32. Preferably, these loads and circuit remainactivated for a predefined time interval (e.g. two to five minutes) asdetermined by a timer within the controller 18. If this time periodelapses without the engine starting, the first and second MOSFETtransistors 34 and 36 are turned off until the key fob is activatedagain by the driver. As a back-up, a manual switch may be provided onthe fuse block or elsewhere in the car to enable the controller 18 toreactivate the car circuits in the event that the key fob is lost orinoperative.

[0030] The present invention has been described with reference tospecific voltage levels and time periods. A skilled artisan willappreciate that these values are a function of the particular batterypowered circuit to which the invention is being applied and by no meansare they the only voltage levels and time periods which can be employed.

We claim:
 1. In a vehicle having an electric motor for starting anengine, an alternator driven by an engine to generate electricity, andaccessory electrical loads; a battery system comprising: a first batteryfor selectively powering the electric motor to start the engine, andhaving a first charge level; a second battery to operate and maintainaccessory electrical loads, and having a second charge level; a chargemaintenance device connecting the first battery to the second batteryfor maintaining a charge of the first battery to a predefined level; anda controller coupled to the first battery to monitor the first chargelevel, and operating the charge maintenance device in response to thefirst charge level.
 2. The battery system as recited in claim 1 whereinthe second battery has an absorptive glass mat type construction.
 3. Thebattery system as recited in claim 2 wherein the second battery has arelatively high reserve capacity and is adapted to provide a relativelylow-rate discharge for an extended period of time.
 4. The battery systemas recited in claim 1 wherein the controller activates the chargemaintenance device when the first battery requires charging while thealternator is not generating electricity.
 5. The battery system asrecited in claim 1 further comprising a charging switch connecting thefirst battery to the alternator and being connected to the controller,wherein the controller operates the charging switch in response tovoltage across the second battery.
 6. The battery system as recited inclaim 5 wherein the controller closes the charging switch in response tothe voltage across the second battery being greater than a firstthreshold, and opens the charging switch in response to the voltageacross the second battery being less than a second threshold.
 7. Thebattery system as recited in claim 6 wherein the second threshold isless than the first threshold.
 8. The battery system as recited in claim1 further comprising a power switch which connects the accessoryelectrical loads to one of the first battery and the second battery, andwhich is connected to the controller, wherein the controller operatesthe power switch in response to voltage across the second battery. 9.The battery system as recited in claim 1 further comprising a powerswitch which connects the accessory electrical loads to the firstbattery, and which is connected to the controller, wherein thecontroller operates the power assembly to disconnect accessoryelectrical loads when voltage across the second battery is below a giventhreshold.
 10. The battery system as recited in claim 1 wherein thecharge maintenance device responds to a control signal from thecontroller by applying a series of currect pulse to recharge the firstbattery.
 11. In a vehicle having an electric motor for starting anengine, an alternator driven by an engine to generate electricity, andaccessory electrical loads; a battery system comprising: a first batteryfor selectively powering the electric motor to start the engine, andhaving a first voltage level and a charge level; a second battery tooperate and maintain accessory electrical loads, and having a secondvoltage level; a charging switch connecting the first battery to thealternator; a charge maintenance device connecting the first battery tothe second battery to maintain the charge level of the first battery toa predefined magnitude in response to a control signal; and a controllercoupled to both the first battery and the second battery to monitor thefirst voltage level and the second voltage level, and operating thecharging switch and the charge maintenance device in response to thefirst voltage level and the second voltage level.
 12. The battery systemas recited in claim 11 wherein the controller produces the controlsignal thereby activating the charge maintenance device when the firstbattery requires charging while the alternator is not generatingelectricity.
 13. The battery system as recited in claim 11 wherein thecontroller closes the charging switch in response to the second chargelevel being greater than a first threshold, and opens the chargingswitch in response to the second charge level being less than a secondthreshold.
 14. The battery system as recited in claim 13 wherein thesecond thresold is less than the first threshold.
 15. The battery systemas recited in claim 11 further comprising a power switch which connectsthe accessory electrical loads to the first battery and which isconnected to the controller, wherein the controller operates the powerswitch in response to the second charge level.
 16. The battery system asrecited in claim 11 further comprising a power switch which connects theaccessory electrical loads to the first battery and which is connectedto the controller, wherein the controller operates the switch assemblyto disconnect accessory electrical loads when the second charge level isbelow a given threshold.
 17. The battery system as recited in claim 11wherein the charge maintenance device responds to the control signal byapplying a series of currect pulse to recharge the first battery.